Most light meters measure the instantaneous photosynthetic photon flux density in µmol m-2 s-1 and yet plant growth is determined by the integrated daily photosynthetic photon flux density in mol m-2 d-1.
Here is how to convert from µmol m-2 s-1 to mol m-2 d-1 when using electric lights.
There are 3600 seconds in an hour. If the electric light output is constant, the instantaneous PPFD can be added up (integrated) over an hour.
Therefore 1 µmol m-2 s-1 = 3600 µmol m-2 h-1.
This must be divided by a million (1,000,000) to convert from micromoles to moles: 3600 µmol m-2 h-1 = 0.0036 mol m-2 h-1.
If the electric lights are on continuously (24 hours), the hourly PPFD must be multiplied by 24 to get the daily PPFD: 0.0036 mol m-2 h-1 = 0.0864 mol m-2 d-1.
There are 86,400 seconds in a day (3600 x 24 hours) so the daily PPFD can be calculated directly from the instantaneous PPFD from the following equation: µmol m-2 s-1 multiplied by 86,400 and divided by 1,000,000 = mol m-2 d-1.
So: 1 µmol m-2 s-1 = 0.0864 mol m-2 d-1 with continuous light
These conversions are accurate only under electric lights. The integrated daily PPFD from sunlight changes during the day so a single measurement cannot be used to estimate daily PPFD.
Daily PPFD Reference Table
*There are 50 to 60 moles per m2 day on sunny summer days in outside areas without any shade.